Add a response target picker to /copy (#39997)

## What changed

- Open a picker for `/copy` with the whole response plus each fenced code block and blockquote from the latest response.
- Label code blocks by language, show content previews, and preserve source whitespace and nested quote Markdown when copying an individual target.
- Keep the picker from disturbing terminal scrollback or queued input, retain clipboard leases, and avoid writing copied content to the session log.

## Testing

- Add coverage for target extraction, picker navigation and cancellation, exact copied content, plan responses, queued input, scrollback preservation, clipboard failures, and session logging.

GitOrigin-RevId: 77a35bc99f4f45932d39914f9a709b30fe8cd7c2

codex-rs/tui/src/slash_command.rs

@@ -99,7 +99,7 @@ impl SlashCommand {
             SlashCommand::Fork => "fork the current chat",
             SlashCommand::App => "continue this session in the Desktop app",
             SlashCommand::Quit | SlashCommand::Exit => "exit Codex",
-            SlashCommand::Copy => "copy last response as markdown",
+            SlashCommand::Copy => "copy the last response, code block, or quote",
             SlashCommand::Export => "export the conversation as markdown",
             SlashCommand::Raw => "toggle raw scrollback mode for copy-friendly terminal selection",
             SlashCommand::Diff => "show git diff (including untracked files)",